A large number of commercial and factory plant roofs are of a flat roof design wherein the roofing material itself is often of a built-up asphalt and in more modern systems of a single ply EPDM elastomeric sheet or membrane. In terms of securing a single ply EPDM membrane to the roof itself. one design utilizes a mechanical ballast system that uses a layer of stone over the membrane. While the ballast system may be least expensive it has a disadvantage of being quite heavy (approximately 10 pounds per square foot) thus requiring a heavy roof support structure and in addition the roof slope cannot exceed 10%.
Adhered roof membrane retention systems suffer from a cost penalty while mechanical fasteners and related fastening systems generally require fixation to the roofing substrate via mechanical fasteners namely membrane penetrating and non-penetrating ones. Each of these types of fasteners has a number of favorable features and each of them is also subject to various drawbacks and disadvantages.
Mechanical fastening systems of the nonpenetrating type generally require fixation of a base member to the roofing substrate followed by a securing of the roof membrane thereto by a top clamping or securing member. Examples of some nonpenetrating type fastening systems for securing a membrane to a roof are shown in U.S. Pat. Nos. 3,426,412; 4,519,175; 4,502,256; 4,617,771; and 4,619,094. Penetrating type fastening systems use various rigid and semirigid members for securing the membrane to the roof. These systems require openings to be formed in the membrane either for inserting a fastening plate therethrough for subsequent attachment to the roof, or formed upon insertion of a fastening anchor or fastener into the roof or lower plate. Examples of some of these prior penetrating systems are shown in the patents set forth below.
U.S. Pat. Nos. 3,918,233 and 4,078,351 disclose the use of a bottom member having a threaded nut which is engaged by a threaded fastener which draws a rigid top plate against a roofing cover located between the two plates, and the use of a single fastener driven through the cover and into the roof.
U.S. Pat. No. 4,074,501 shows another penetrating mechanical fastener in which a single threaded member extends through the roof substrate for clamping a disc-shaped member against a waterproof covering sheet.
A somewhat similar fastener disc is shown in U.S. Pat. Nos. 4,467,581 and 4,476,660 in which the disc-shaped top plate is secured to the roof structure by a linearly extending fastener for maintaining a flexible membrane on the roof.
U.S. Pat. No. 4,620,402 discloses a fastening system in which a rigid lower retainer plate is inserted through an opening formed in the membrane after the membrane has been spread over the roof. The plate then is secured to the roof by one or more fasteners. The membrane is compressed against the anchored lower plate and secured thereto by a rigid upper plate which is clamped to the lower plate by a separate fastener.
Still another type of penetrating roof fastener is shown in U.S. Pat. No. 4,520,606. This fastener consists of a base plate which is secured by separate fasteners to the roof substrate. A membrane is placed over the anchored base plate and clamped thereagainst by a top retainer plate or disc which is secured to the base plate by a separate linear fastener. The fastener draws the top plate into a compressing relationship with the lower plate compressing the membrane therebetween. Spacers are then provided for receiving a mastic material to prevent leakage.
Although both penetrating and nonpenetrating type fastener systems do work satisfactory for many applications, it is desirable to have a penetrating system for certain applications. One problem with nonpenetrating type fasteners is that it is difficult to provide a secure engagement between the top and bottom retainer plates to trap the membrane therebetween should the fastener be located at a splice in the roofing membrane since these splicers generally have a double membrane thickness. Therefore, it is desirable for a roof fastener to be able to satisfactorily maintain the roofing membrane secured to the roof even if the fastener is located at a splice in the membrane.
Another problem that exists with certain of the roof fastening systems is the relative ease with which vandals can remove the exposed locking component or top plate of the fastener. Although this is not a problem for many installations, since the roofs are inaccessible to outsiders, it can be a problem for those buildings that are readily accessible and for buildings subject to vandalism such as schools. In many known fasteners, the locking cap or top plate component can be pried off or unscrewed and easily removed requiring only a screwdriver or other readily available tool. It is also desirable in both nonpenetrating and penetrating systems that the fastener be installed in a minimum amount of time and without requiring skilled labor, and that the fastening elements can be securely retained in clamping engagement with the trapped membrane to reduce the accidential disengagement thereof upon the membrane experiencing severe uplift wind forces. These uplift wind forces at times become substantial on the individual fasteners and occasionally can result in the membrane stretching and pulling out from between the clamped fasteners. It has been found that a relatively flat clamping surface between abutting fastening elements or plates provides less resistance to the pulling out of the membrane therebetween than if a changing surface contour is provided for the clamping area. However, no sharp corners or edges can be present on the fastener members since they damage the membrane.
Another problem that exists with present roof fastening systems is that for most fasteners, the top plate should be drawn into clamping engagement with the bottom plate with a particular clamping or compression force on the membrane to prevent damaging the membrane or from damaging either of the retainer plates, or from effecting the anchoring stability of the lower retainer plate on the roof. Still another problem that exists in such dual retainer plate fastener systems is that the uplifting wind forces exerted on the lower plate will cause the anchoring member to back out or loosen from its anchoring in the roof. The lower plate will ultimately become completely separated from the roof structure or the anchoring fastener will pierce the membrane thus causing a leak.
The problem of providing the proper amount of compression force on the membrane is recognized in U.S. Pat. No. 4,545,270 which attempts to solve it by providing a clamping washer with a crushable region in the load distributing flange portion thereof. This crushable region insures that the joint is loaded with predetermined force to reduce the likelihood of overdriving or underdriving the fastener. Still another problem with prior art fastener systems is that many of them require a mastic sealing material to insure a leakproof seal which is both costly and time consuming.